This application claims Paris Convention priority of Japanese patent application number 2000-136606 filed on May 10, 2000, the complete disclosure of which is hereby incorporated by reference.
A. Field of the Invention
The present invention relates to a hydrodynamic bearing device in which a dynamic pressure is generated in a lubricant filled between a dynamic pressure generating surface on a shaft member and a dynamic pressure generating surface on a bearing sleeve to relatively rotate and support both members and, more specifically to a structure and method of fixing a covering member which prevents the leakage of the lubricant.
B. Description of the Background of the Invention
In recent years, various proposals have been made concerning a hydrodynamic bearing device for supporting high speed rotation of various rotary bodies such as magnetic disks, optical disks, polygonal mirrors. FIG. 4 shows an enlarged major portion of a hydrodynamic bearing device. In the hydrodynamic bearing device, a dynamic pressure generating surface 11 on a shaft member (rotary shaft) 10 and a dynamic pressure generating surface 21 on a bearing sleeve 20 are opposed to one another via a predetermined gap. Dynamic pressure generating grooves (not illustrated) are cut on at least one of the dynamic pressure generating surfaces 11 and 21 which are opposed to each other. A lubricant 6 such as oil and air is filled in the gap. As the rotary body rotates, a pressure is applied to the lubricant 6 through a pumping action of the dynamic pressure generating grooves to generate a dynamic pressure, with which the shaft member 10 and the bearing sleeve 20 are rotatably supported in a relatively non-contact manner.
A ring-like thrust plate 30 is fitted to an end of the shaft member 10 and stored inside a storing portion 23 which has a diameter larger than the inner diameter of the bearing surface 21 on the bearing sleeve 20. At the opening portion of the bearing sleeve 20, a disk-like covering member 40 tightly abuts to an outer surface of a step 22 provided to create the storing portion 23 and is fixed by an adhesive 70 and a caulking portion 24.
Thrust dynamic pressure generating grooves (not illustrated) are cut on the axial end surfaces of the thrust plate 30 or the end surfaces of the covering member 40 and bearing sleeve 20 which face the axial end surfaces of the thrust plate 30 so that a dynamic pressure is generated in the lubricant 6 as the thrust plate 30 rotates. With this, the thrust plate 30, the bearing sleeve 20, and the covering member 40 are supported in a non-contact manner. Note that the code 50 indicates a frame holding the bearing sleeve with the inner circumferencial surface thereof.
In such a hydrodynamic bearing device, it is important to prevent the lubricant 6 therein from leaking to the outside of the device. One of the general countermeasures is shown in FIG. 4. The covering member 40 and the bearing sleeve 20 are joined together by an adhesive 70 applied in a radial gap S1 therebetween to prevent the leakage of the lubricant 6.
However, when a normal adhesive is applied to join the covering member 40 and the bearing sleeve 20 in a hydrodynamic bearing device, it causes the following problems. Since the surfaces of the covering member 40 and bearing sleeve 20 are finished by a lathe, fine cutting marks are left thereon. Even if the step portion 22 of the bearing sleeve 20 and the covering member 40 are put together, there is a sub-micron order gap S2 therebetween in the thrust direction. Therefore, as the adhesive 70 is applied in the gap S1, it permeates to the gap S2 with capillary action.
As a result, the covering member 40 becomes slightly raised with respect to the step portion 22 of the bearing sleeve 20, which changes the height inside the storing portion 23, and therefore, the gap between the thrust plate 30 and the shaft member 10 in the thrust direction varies to a great extent. Also, the adhesive 70 that has permeated into the gap S2 between the step portion 22 of the bearing sleeve 20 and the covering member 40 may diffuse to the inside the device and mix in the lubricant 6. If the uncured adhesive 70 is mixed in the lubricant 6, the viscosity of the lubricant 6 greatly increases. This increases the rotational load, which requires more power.
Considering the above problems, an objective of the present invention is to provide a hydrodynamic bearing device in which a covering member is fixed more firmly by an adhesive which can be hardened for sure, so that the lubricant is kept from leaking and the adhesive from permeating into the lubricant.
In accordance with one embodiment of the invention, a hydrodynamic bearing device is disclosed which comprises: a cylindrical bearing sleeve which has a first dynamic pressure generating surface; a shaft member which has a second dynamic pressure generating surface and is attached relatively rotatable to the bearing sleeve; a lubricant filled in a gap between the first and second dynamic pressure generating surfaces; a covering member which is fixed by adhesive to one of the openings of said bearing sleeve to prevent the lubricant from leaking; and an adhesive which is applied in a joining portion between the bearing sleeve and said covering member to seal a gap therebetween; wherein the adhesive is a two-part liquid epoxy resin adhesive that satisfies the condition, n/2xe2x89xa6mxe2x89xa62n, where m is the molecular-weight of a primary agent and n is the molecular-weight of a curing agent.
In accordance with another embodiment of the present invention, a hydrodynamic bearing device is disclosed which comprises: a cylindrical bearing sleeve which has a first dynamic pressure generating surface; a frame which has a sleeve holding portion for holding said bearing sleeve; a shaft member which has a second dynamic pressure generating surface and is attached relatively rotatable to the bearing sleeve; a lubricant filled in a gap between the first and second dynamic pressure generating surfaces; a covering member which is fixed by adhesive to one of the openings of the frame to prevent the lubricant from leaking; and an adhesive which is applied to a joining portion between the bearing sleeve and the covering member to seal a gap therebetween; wherein the adhesive is a two-part liquid epoxy resin adhesive that satisfies the condition, n/2xe2x89xa6mxe2x89xa62n, where m is the molecular-weight of a primary agent and n is the molecular-weight of a curing agent.
Although in present invention the covering member is joined to a different covering member, the bearing sleeve or the frame, the same adhesive is used to join them. The two-part liquid epoxy resin, in which the molecular-weight of the primary agent is relatively close to the molecular-weight of the curing agent, is used to join the covering member and the counter member; therefore, the primary agent and the curing agent diffuse at the same time into the gap between the covering member and the counter member (the gap in the thrust direction and/or radial direction). This causes a curing reaction in the gap for sure. Consequently this prevents the adhesive from remaining uncured and from diffusing into the lubricant. Thus, the covering member is firmly secured by the adhesive, and the adhesive is kept from leaking to the outside of the hydrodynamic bearing device.
In the hydrodynamic bearing device of the present invention, it is preferred that the primary agent and the curing agent are substantially equivalent in the molecular-weight. With this, the primary agent and the curing agent diffuse in any gap under the same condition, and the invention of claim 1 or 2 becomes more effective.
In the hydrodynamic bearing device of the present invention, it is ideal to further use a joining means by caulking, screwing, or welding for joining the covering member and the counter member. With this, the joining strength between the covering member and the counter member is increased, improving shock resistance of the hydrodynamic bearing device.